Phosphorylation of proteins is a fundamental mechanism for regulating diverse cellular processes. While the majority of protein phosphorylation occurs at serine and threonine residues, phosphorylation at tyrosine residues is attracting a great deal of interest since the discovery that many oncogene products and growth factor receptors possess intrinsic protein tyrosine kinase activity. The importance of protein tyrosine phosphorylation in growth factor signal transduction, cell cycle progression and neoplastic transformation is now well established (Hunter et al., Ann. Rev. Biochem. 54:987-930 (1985), Ullrich et al., Cell 61:203-212 (1990), Nurse, Nature 344:503-508 (1990), Cantley et al., Cell 64:281-302 (1991)).
Biochemical studies have shown that phosphorylation on tyrosine residues of a variety of cellular proteins is a dynamic process involving competing phosphorylation and dephosphorylation reactions. The regulation of protein tyrosine phosphorylation is mediated by the reciprocal actions of protein tyrosine kinases (PTKases or PTKs) and protein tyrosine phosphatases (PTPs). The tyrosine phosphorylation reactions are catalyzed by PTKs. Tyrosine phosphorylated proteins can be specifically dephosphorylated through the action of PTPs. The level of protein tyrosine phosphorylation of intracellular substances is determined by the balance of PTK and PTP activities. (Hunter, T., Cell 58:1013-1016 (1989)).